The present invention relates generally to the regulation of power to certain loads which are insensitive to rapid fluctuation of power and, more particularly to a system which controls the conduction period so that power to a load, is applied and removed, at a controlled (slowed-down) rate.
Systems which include application of ac line voltage to loads such as heavy filament tungsten lamps, heater circuits or the like, include components which compensate for the inability of these loads to respond to the rapid power fluctuation of the line source. Various voltage regulation techniques are known in the art; the most commonly being to electrically apply and remove the load power by selectively controlling the operation of a silicon-controlled-rectifier (SCR). The SCR operation, in turn, is usually controlled by using an optical feedback circuit. Systems of this type are disclosed in U.S. Pat. Nos. 3,210,647, 3,952,242 and 3,670,202.
Each of these systems, however, encounters the problem of compensating for the effects of electromagnetic interference (EMI) caused by the rapid switching times associated with SCRs (typically 1 to 2 .mu.s). The EMI effects are manifested in fast rise times in the load voltage which must be eliminated. One method of eliminating the EMI effect is to insert filters between the SCR and the load as shown, for example, in U.S. Pat. No. 3,670,202. This represents an additional, and costly, component to these systems and increases their complexity. The use of filters also creates an undesireable AC ground leakage current which must be limited for safety reasons.
The present invention is directed to a power regulation system which applies and removes power from a load "slowly", thereby reducing the EMI and eliminating the need for filters. A transistor, rather than an SCR, is then used to control the rate at which power is applied to the load. According to one aspect of the invention, there is provided: rectifier means for converting alternating current power to full wave rectified direct current power; means for supplying said rectified power to the load; means for generating a first output signal V.sub.c representative of the radiant energy of said load; means for comparing said output V.sub.c with a portion of said rectified dc power to produce a second output V.sub.o ; a transistor switching circuit coupled between said comparing means and said load, said switching circuit being adapted to be turned on and off at a rate determined by the rate of change of the level of output V.sub.o ; whereby said load is turned on and off consistent with the switching rate of said transistor switching circuit.